Biocarbon, char, biochar or charcoal is an important compound, since the product has a number of uses, examples of which include fuel and reactant in the refining of metals. Typically, pyrolyzable organic materials are attractive progenitors for biocarbon production. Recently, new uses for biocarbons have become particularly important. One of these is the use of biocarbon for the sequestration of carbon dioxide, a greenhouse gas, and the addition to soils as an amendment to improve soil fertility. There is, given the diversity of product utilizations, a need to have efficient methods for formulating customizable types of biocarbons from pyrolyzable organic materials, such as biomass.
Thermal conversion of organic compounds into derivative substances that contain greater relative portions of carbon is known as pyrolysis or carbonization. It is well known that the formation of the charcoal typically involves the liberation of the volatile organic compounds from pyrolyzable organic materials, in particular, the thermal conversion of lignocellulose material. Depending on the specific thermal conversion process, supplemental thermal energy may be required or excess thermal energy may have to be removed to properly control the thermal conversion.
The addition of thermal energy can be achieved by a number of commonly practiced techniques such as heat exchange or oxidation of available organic matter. A common method of oxidation of available organic matter is to provide a limited source of oxygen, typically in the form of air, to the pyrolyzing organic materials, thereby concurrently oxidizing some of the available organic matter and releasing the associated thermal energy within the remaining solids. The removal of excess thermal energy can also be achieved by both direct and indirect heat exchange, where inert cooler gases are introduced into the vapor phase of the pyrolysing organic matter.
The art has developed substantially over the years and the improvements that are presently known are basically predicated on the Lambiotte retort. The Lambiotte retort is a classical vertical shaft furnace. The basic features include counter-current flow of solids (down) and vapors (up), with addition of air to provide internal heating via partial oxidation of the pyrolyzable organic matter, typically wood. The design is dated; however, the apparatus is efficient at producing large quantities of charcoal at relatively low conversion efficiency of the raw material. The benefits of the system relate to relatively light labour requirements with high mechanical reliability.
Subsequent to the introduction of the Lambiotte retort and the process for operating the retort, numerous other patents have issued directed to improvements in apparatus and methodology to effect the production of the charcoal. As an example, two related U.S. patents include U.S. Pat. No. 5,725,738, issued Mar. 10, 1998 to Brioni et al. and U.S. Pat. No. 5,882,484, issued Mar. 16, 1999 to Pyy. Both feature the method of compartmentalizing the entering pyrolyzable organic materials into totes and progressing the solids sequentially in the totes through the drying and thermal conversion stages.
In U.S. Pat. No. 3,962,045, issued Jun. 8, 1976 to Douglas et al., there is disclosed a pyrolyzing vessel having inlets and outlets for the fuel charged. The pyrolyzing gas passes through the vessel along paths at substantially right angles. The patent also references the fact that effective heat transfer from the gas phase to the solids requires an excessive gas flow to the solids throughput on a weight basis. It is indicated in column 3 at lines 33 et. seq., “a typical maximum gas/refuse rate would be 10:1 by weight, that is to say 10 lbs of gas circulating for every 1 lb of raw refuse pyrolysed”. This reference is useful for teaching the fact that vapor travel perpendicular to the direction of the solids movement allows for a lower pressure drop by having a shorter path through the bed of solids.
In respect of further developments in the art, in U.S. Pat. No. 5,435,983, issued Jul. 25, 1995, to Antal, there is disclosed a biocarbon producing method. The process establishes the merits of using controlled combustion as a source of heat. In the reference, heat is provided by both oxidation of air in the reactor and external heating. In contrast, in Antal's later patent, U.S. Pat. No. 6,790,317, issued Sep. 14, 2004, the same basic reactor is operated in a mode where the reactor is pressurized and the bottom of the biomass is ignited, then additional air supplied to sustain the partial combustion processes to generate the necessary heat for the conversion of biomass charge to charcoal. In comparative examples 5 and 6 of this reference, Antal establishes the lower energy requirement of his “reverse burn” approach and the absence of any appreciable loss of yield. Antal further establishes that the temperature range of between 400° C. and 600° C. for the highest temperature measured in the reactor, which is a lower range of temperature experienced in earlier processes where the char is heated to higher temperatures, often above 1000° C. Although significant features are taught in the references, a disadvantage of this patentee's approach relates to the batch mode of operation and the high cost of providing a pressurized reactor controlling a non-steady state process. In the literature, Antal establishes that his process does not create a homogenous product with the material at the bottom, middle and top of the reactor having physical and chemical property variations that cannot be eliminated (Ind. Eng. Chem. Res. 2003, 42, 3690-3699 and Ind. Eng. Chem. Res. 2006, 45, 585-599).
Additional improvements in the overall processing of charcoal are further recognized in Ind. Eng. Chem. Res. 2003, 42, 1619-1640, where Antal discussed the following:
“Vapor-Phase Residence Time. Although Klason and established the key role of secondary (vapor-phase) pyrolytic reactions in the formation of charcoal 88 years ago; today on, many researchers still assume that the charcoal is solely a product of primary (solid-phase) pyrolytic reactions. In reality, charcoal contains both “primary” charcoal and “secondary” charcoal that is a coke derived from the decomposition of the organic vapors (“tars”) onto the solid carbonaceous solid. This decomposition is probably catalyzed by the charcoal.”
The importance of secondary charcoal formation appears in U.S. Pat. No. 4,145,256, issued Mar. 20, 1979 to Bowen, where it is described in the abstract: “At the same time, a temperature gradient is established in the reaction zone which will allow some of the vapor component of the decomposition (i.e. heavier tars) to condense on the solid component at cooler regions for subsequent reintroduction into the high temperature regions so that the latent carbon content of these tars may also be recovered. The throughput or residence time of the material through the reaction zone and the amount of air introduced (to control the maximum reaction temperature as well as the temperature gradient) are controlled in related manner to recover more or less of the heavier tar residue.”
The actual reactor configuration of U.S. Pat. No. 4,145,256 is a basic counter-current flow, with the vapors travelling up through the bed of descending solids, as found in the Lambiotte retort. As such, the extent of control over the recovery of heavier tars and the flexibility of controlling the temperature profile within the bed by controlling the amount of air injected were limited. However, U.S. Pat. No. 4,145,256 does place an appropriate emphasis on the value of secondary charcoal formation from heavier tars and the importance of providing suitable control over the temperature profile during the conversion of the entering pyrolyzable organic materials into a high quality biocarbon product.